- Phosphate, silicate and nitrate concentrations in the sea off
the coast of Oregon were determined to study their seasonal distribution
patterns. The water samples were collected monthly along the
east-west hydrographic line off Newport, Oregon, from the shore to
165 miles (310 km) offshore. The samples were analyzed by both a
Technicon Autoanalyzer® and by manual methods.
Phosphate in January was greater than 0.5μM at the surface,
increasing to a 3.3μM maximum at 1000m. A phosphate maximum
occurred at 1000m throughout the year. With the onset of photosynthetic
activity, there was a general decrease in surface concentrations.
Concentrations decrease to less than 0.1μM in October
with surface concentrations increasing in November and December,
reaching a maximum in January.
Silicate concentration was approximately 170μM at 2600m during
the entire year. In January, surface silicate increased from less than
5μM offshore to 13μM nearshore; a weak silicate minimum was
observed at a depth of 60m offshore and at 5m near the coast. In
March nearshore surface concentrations were greater than 15μM due
to increased coastal river runoff; the silicate concentrations of
Oregon coastal rivers were 150-250μM. In July, a pronounced minimum
occurred at 40-50m depth, where the concentration was less
than 2μM. Summer surface concentrations were less than 2M offshore,
increasing to 40μM nearshore as a result of coastal upwelling.
Surface concentrations in October were generally less than 5FIM along
the Newport hydrographic line with increased near shore concentrations
due to river runoff.
The nitrate maximum of 40-45μM existed at 1000- 1200m. In
January surface concentrations ranged from 3μM offshore to 6μM
nearshore. March values were lower, ranging from 11μM offshore to
5μM nearshore at NH-3 (6 km). In July patches of nitrate-free water
were observed at 40-50m depth offshore, disappearing at 25 miles
(46 km) off the coast, at station NH-25, as a result of the influence
of upwelled and nitrate containing water. Late in October, nitrate
concentrations were higher, 5-6μM, near the coast, and dropping to
less than 0.1μM offshore. Patches of nitrate-free water were found
from May to November.
The existence of nitrate-free water near the surface indicated
that the feature was formed by the photosynthetic activities of phytoplankton.
The formation of the silicate minimum and oxygen
maximum below or near the bottom of the Columbia River Plume
during summer months indicated intense photosynthetic activity.
Neither preformed phosphate nor preformed nitrate changed
with depth below the layers of active photosynthetic processes. Preformed
phosphate was generally 1.0-1.2μM below 200m with variation
occurring in surface waters. For surface values, 1.2μM, the highest
of the year, was observed in July in upwelled water; 0.4μM, the
lowest, also occurred in July within the influence of the Columbia
The vertical distribution of preformed nitrate seemed to be
random and may have reflected analytical techniques. The range was
between 5 and 10μM with a maximum at 12μM. The 5μM values were
found at approximately 200m with a maximum observed at 1000m
deep. The highest surface values of preformed nitrate, 2.0-5.0μM,
were observed in January and the lowest, 1.0-1.5μM, in July. March
and October values were between the above extremes (1.1-2.0μM).